As optical communications systems are widely deployed, there is an increasing need for devices capable of combining, separating, switching, adding and dropping optical signals. For example, broadband optical multiplexers are needed for delivering voice and video signals to the home, for combining pump and communications signals in an optical amplifier, and for adding monitoring signals to optical fibers. Dense wavelength-division multiplexing (WDM) systems need multiplexers to combine and separate channels of different wavelengths and need add-drop filters to alter the traffic. Low speed optical switches are needed for network reconfiguration.
These important functions are typically performed by optical waveguide devices such as integrated optical silica waveguide circuits formed on planar silicon substrates. Such waveguides are typically formed by depositing base, core and cladding layers on a silicon substrate. The base layer can be made of undoped silica. It isolates the fundamental optical mode from the silicon substrate and thereby prevents optical loss at the silica substrate interface. The core layer is typically silica doped with phosphorus or germanium to increase its refractive index and thereby achieve optical confinement. The cladding is typically silica doped with both boron and phosphorus to facilitate fabrication and provide an index matching that of the base. Using well-known photolithographic techniques, the cores can be economically configured into a wide variety of compact configurations capable of performing useful functions. See, for example, Y. P. Li and C. H. Henry, "Silicon Optical Bench Waveguide Technology", Ch. 8, Optical Fiber Telecommunications, Vol. IIIB, p. 319-375 (Academic Press, 1997).
Other waveguide devices are made of optical fiber. Optical fibers typically comprise a higher index core, which can be doped silica, and a surrounding cladding of a lower index glass. A variety of all-fiber devices are made by providing one or more Bragg gratings in the fiber core. Such gratings are conventionally made by providing the core with a photosensitive dopant such as germanium and side-writing a grating using ultraviolet light.
One shortcoming of these optical waveguide devices is their sensitivity to temperature. Many waveguide devices are based upon optical interference between beams of light propagated down different paths. Depending on the phase relationship between the beams at the point of recombination, light will either be transmitted or reflected back. Spectrally narrow, high contrast resonances can be readily designed, enabling high performance wavelength division multiplexers and blocking filters. However variable ambient temperature has a perceptible and disadvantageous effect on the performance of such devices. The refractive index of the composite glass structure through which the light travels depends on temperature. Thus the spectral positions of critical resonances shift with temperature.
Similar problems occur in fiber waveguide devices. Bragg gratings, for example, are critically dependent on the path lengths between successive index perturbations. But these path lengths change due to the temperature dependence of the refractive index, shifting the operating wavelength of the gratings.
For many applications such variation is not acceptable, and the devices are placed in temperature compensating packages for stable operation. Such packaging is expensive and adds reliability problems. Accordingly there is a need for waveguide devices having enhanced temperature stability.